1. Field of the Invention
The present invention relates to a piezoelectric element, and more specifically, to a piezoelectric element which is easy to mount on or in a piezoelectric device, such as a piezoelectric pump. The present invention further relates to a piezoelectric device including the piezoelectric element.
2. Description of the Related Art
A piezoelectric element has recently been used as a drive source of a small-sized, high-precision pump, blower, fan, or other suitable device. Further, the piezoelectric element for this purpose is required to be capable of being driven with a relatively low voltage and obtaining a large displacement amount at a central portion of the piezoelectric element, and not to be degraded in characteristics over time.
These demands are satisfied by, for example, a piezoelectric element for a drive source of a piezoelectric pump disclosed in WO 2008/007634.
FIGS. 10 and 11 illustrate a piezoelectric element 400 disclosed in WO 2008/007634. FIG. 10 is a perspective view, and FIG. 11 is an exploded perspective view.
The piezoelectric element 400 includes a piezoelectric body 101 including a plurality of laminated piezoelectric layers 101a to 101h. A first terminal 111, a second terminal 112, and a third terminal 113 are arranged in a line on surfaces of the piezoelectric body 101.
A rectangular ground electrode 121 is provided on a surface of the piezoelectric layer 101a defining the uppermost layer. The ground electrode 121 is extracted to the second terminal 112.
A circular central electrode 124 is provided in a central portion of a surface of the next piezoelectric layer 101b, and a ring-shaped peripheral electrode 125 is provided therearound. The central electrode 124 is extracted to the third terminal 113, and the peripheral electrode 125 is extracted to the first terminal 111.
A rectangular ground electrode 121 is provided on a surface of the next piezoelectric layer 101c. The ground electrode 121 is extracted to the second terminal 112.
A circular central electrode 124 is provided in a central portion of a surface of the next piezoelectric layer 101d, and a ring-shaped peripheral electrode 125 is provided therearound. The central electrode 124 is extracted to the third terminal 113, and the peripheral electrode 125 is extracted to the first terminal 111.
A rectangular ground electrode 121 is provided on a surface of the next piezoelectric layer 101e. The ground electrode 121 is extracted to the second terminal 112.
A circular central electrode 124 is provided in a central portion of a surface of the next piezoelectric layer 101f, and a ring-shaped peripheral electrode 125 is provided therearound. The central electrode 124 is extracted to the first terminal 111, and the peripheral electrode 125 is extracted to the third terminal 113.
A rectangular ground electrode 121 is provided on a surface of the next piezoelectric layer 101g. The ground electrode 121 is extracted to the second terminal 112.
A circular central electrode 124 is provided in a central portion of a surface of the next piezoelectric layer 101h, and a ring-shaped peripheral electrode 125 is provided therearound. The central electrode 124 is extracted to the first terminal 111, and the peripheral electrode 125 is extracted to the third terminal 113. Further, the rear surface of the piezoelectric layer 101h is provided with a rectangular ground electrode 121. Furthermore, the ground electrode 121 is extracted to the second terminal 112.
The existing piezoelectric element 400 includes the ground electrodes 121 exposed on both upper and lower surfaces thereof. However, the piezoelectric element 400 may be configured such that piezoelectric layers which do not include an electrode are laminated on both the upper and lower surfaces so as not to expose the ground electrodes 121.
The piezoelectric body 101 having the above-described structure is formed by a method of, for example, laminating, compressing, and firing piezoelectric green sheets for forming the piezoelectric layers 101a to 101h, on which the necessary electrodes (the ground electrodes 121, the central electrodes 124, and the peripheral electrodes 125) are formed by conductive paste or the like. Further, the first terminal 111, the second terminal 112, and the third terminal 113 are formed by a method of, for example, baking conductive paste onto surfaces of the fired piezoelectric body 101.
As illustrated in FIGS. 12A and 12B, for example, the piezoelectric element 400 is polarized by a direct-current voltage applied to the first terminal 111, the second terminal 112, and the third terminal 113. FIGS. 12A and 12B are explanatory diagrams illustrating a polarization process of the piezoelectric element 400. FIG. 12B illustrates a portion of FIG. 12A indicated by a dash-dotted line X-X. In FIG. 12B, the scale in the height direction is enlarged as compared to that in the width direction to clarify polarization directions.
As illustrated in FIG. 12A, the first terminal 111, the second terminal 112, and the third terminal 113 are applied with a negative voltage, a ground voltage, and a positive voltage, respectively. As a result, as illustrated in FIG. 12B, each of the piezoelectric layers 101a to 101h of the piezoelectric element 400 is polarized in the directions indicated by arrows in the drawing. As understood from FIG. 12B, in each of the piezoelectric layers 101a to 101h, the polarization direction is opposite between a central portion and a peripheral portion. Further, due to the difference in extraction of the central electrodes 124 and the peripheral electrodes 125 to the first terminal 111 and the third terminal 113, the repeating order of polarization is different between the piezoelectric layers 101a to 101d and the piezoelectric layers 101e to 101h. 
The piezoelectric element 400 having such a structure and subjected to polarization is used as, for example, a drive source of a piezoelectric device, such as a piezoelectric pump. FIGS. 13A and 13B illustrate a piezoelectric pump 500 including the piezoelectric element 400 as a drive source thereof. FIG. 13A is a perspective view, and FIG. 13B is a cross-sectional view illustrating a portion of FIG. 13A indicated by a dash-dotted line Y-Y.
The piezoelectric pump 500 includes a pump body 131. The pump body 131 includes a hollow pump chamber 131a, an opening 131b that opens an upper portion of the pump chamber 131a, and two holes 131c and 131d that communicate with the pump chamber 131a. 
The opening 131b is closed by a diaphragm 132, and the piezoelectric element 400 is bonded and fixed to the diaphragm 132.
Further, an inflow check valve 133 is attached to the hole 131c, and an outflow check valve 134 is attached to the hole 131d. The inflow check valve 133 functions to allow fluid to flow into the pump chamber 131a from the outside but prevent fluid from flowing out in the opposite direction. The outflow check valve 134 functions to allow fluid to flow to the outside from the pump chamber 131a but prevent fluid from flowing in the opposite direction.
Further, three metal terminal members 141, 142, and 143 are fixed on the upper surface of the pump body 131. Further, the metal terminal members 141, 142, and 143 are connected to the first terminal 111, the second terminal 112, and the third terminal 113 of the piezoelectric element 400, respectively, by a bonding material, such as conductive paste or solder. The illustration of the boding material is omitted in FIG. 13A.
FIGS. 14A to 14C illustrate a drive state of the piezoelectric pump 500. FIGS. 14A to 14C are explanatory diagrams. FIGS. 14B and 14C illustrate a portion of the piezoelectric element 400 in FIG. 14A indicated by a dash-dotted line Z-Z. In FIG. 14B, the scale in the height direction is enlarged as compared to that in the width direction to clarify the expansion and contraction of the piezoelectric layers 101a to 101h defining the piezoelectric element 400.
As illustrated in FIGS. 14A and 14B, to drive the piezoelectric pump 500, the piezoelectric element 400 includes an alternating-current power supply serving as a drive power supply and including one terminal connected to the second terminal 112 via the metal terminal member 142 and the other terminal connected to the first terminal 111 and the third terminal 113 via the metal terminal member 141 and the metal terminal member 143, respectively. The alternating-current power supply is not limited to the one that generates sine waves, and may be the one that generates, for example, rectangular waves.
The piezoelectric layers 101a to 101h forming the piezoelectric body 101 of the piezoelectric element 400 are polarized, as illustrated in FIG. 12B. When applied with an alternating-current voltage, therefore, the piezoelectric layers 101a to 101h partially expand or contract at some point of time, as indicated by arrows in FIG. 14B, for example. At this point of time, for example, a central portion of the piezoelectric layers 101a to 101d contracts, and a peripheral portion located therearound expands. Meanwhile, a central portion of the piezoelectric layers 101e to 101h expands, and a peripheral portion located therearound contracts. As a result, the piezoelectric element 400 includes a central portion downwardly bent and displaced and a peripheral portion upwardly bent and displaced, as indicated by hatched arrows in FIG. 14C.
Then, if the polarity of the alternating-current power supply changes, the piezoelectric element 400 exhibits an opposite behavior to that illustrated in FIG. 14B and FIG. 14C. That is, the central portion of the piezoelectric layers 101a to 101d expands, and the peripheral portion located therearound contracts. Further, the central portion of the piezoelectric layers 101e to 101h contracts, and the peripheral portion located therearound expands. As a result, the piezoelectric element 400 includes the central portion upwardly bent and displaced and the peripheral portion downwardly bent and displaced.
Applied with the alternating-current voltage, the piezoelectric element 400 repeats these behaviors. Thereby, in the piezoelectric pump 500 illustrated in FIGS. 13A and 13B, fluid flows into the pump chamber 131a through the hole 131c, and the fluid flows into the pump chamber 131a flows to the outside through the hole 131d. 
In the existing piezoelectric element 400 having the above-described configuration, the piezoelectric body 101 includes multiple layers of the piezoelectric layers 101a to 101h, and each of the piezoelectric layers 101a to 101h is polarized in the opposite directions between the central portion and the peripheral portion. It is therefore possible to drive the piezoelectric element 400 with a relatively low voltage, and to obtain a large displacement amount at the central portion of the piezoelectric element 400. Accordingly, a piezoelectric device (such as the piezoelectric pump 500) including the piezoelectric element 400 as a drive source thereof efficiently functions with low power consumption.
Further, the existing piezoelectric element 400 has functions of preventing, when in use, electromigration between the central electrode 124 and the peripheral electrode 125 provided on the same layer, and not being degraded in characteristic over time. That is, there was an issue that, if a central electrode and a peripheral electrode provided on the same layer are different in potential when is use, electromigration may occur between the central electrode and the peripheral electrode after a certain period of usage, and cause a short circuit therebetween and the degradation of characteristics or the breakage of a piezoelectric element. This issue was serious particularly when Ag was used as a main component of the central electrode and the peripheral electrode.
Meanwhile, when the piezoelectric element 400 is in use, the alternating-current power supply has one terminal connected to the second terminal 112 and the other terminal connected to the first terminal 111 and the third terminal 113, as illustrated in FIGS. 14A and 14B, with the first terminal 111 and the third terminal 113 constantly maintained at the same potential. As a result, all of the central electrodes 124 and the peripheral electrodes 125 in the piezoelectric element 400 are constantly maintained at the same potential. Thus, electromigration does not occur between the central electrode 124 and the peripheral electrode 125 provided on the same layer, and no short circuit occurs therebetween. The piezoelectric element 400 does not suffer the degradation of characteristics over time and breakage due to electromigration.
As described above, the existing piezoelectric element 400 disclosed in WO 2008/007634 has excellent features of being capable of being driven with a relatively low voltage and obtaining a large displacement amount at the central portion of the piezoelectric element, and not being degraded in characteristics over time.
However, the piezoelectric element 400 has an issue that, when used as a drive source of a piezoelectric device, such as a piezoelectric pump, all of the first terminal 111, the second terminal 112, and the third terminal 113 need to be connected to the terminals of the alternating-current power supply (drive power supply), and thus, that the degree of design freedom of the piezoelectric device is restricted and the manufacturing of the piezoelectric device is complicated.
That is, in the piezoelectric element 400, the first terminal 111 and the third terminal 113 are constantly applied with a voltage of the same potential from the alternating-current power supply, as illustrated in FIGS. 14A and 14B. However, the second terminal 112 is disposed between the first terminal 111 and the third terminal 113. Therefore, the first terminal 111 and the third terminal 113 need to be connected individually to a terminal of the alternating-current power supply. For this reason, there is an issue that a piezoelectric device, such as a pump, including the piezoelectric element 400 as a drive source thereof is restricted in the degree of design freedom and is complicated to manufacture.